1. Field of the Invention
The present invention relates to a method of injection molding a cross-linked foamed composition. In particular, the present invention relates to the use of inert gas to evacuate oxygen from hot molds to minimize deleterious effects during molding, such as oxidation. The present invention further includes an apparatus to perform this method of injection molding.
2. Related Art
Thermoplastic materials are often molded by the process of injection molding. Conventionally, a polymer is preheated in a cylindrical chamber to a temperature at which it will flow and then forced into a relatively cold, closed mold chamber under high pressures to distribute the material without voids. Once the polymer melt has solidified in the cool mold, the mold is opened and the molded article is removed. Such methods, however, are not conducive to the production of foamed cross-linked polymers.
Foamed cross-linked polymers are produced by generating a gas in a fluid polymeric material, usually within a hot mold maintained at an elevated temperature. Thermoplastics may be foamed by incorporating a chemical blowing agent, which decomposes to a gas at elevated temperature.
Inert gas at a high pressure, for example, 500 psi and greater, provides back-pressure upon the fluid polymeric material to prevent initial foaming and oxidation. The pressure is then released and the article foams quickly. This process is more complicated because it requires the use of higher pressure machine tooling which requires increased maintenance. Furthermore, this process is fairly inaccurate. The final dimensions vary because it is difficult to accurately control the foaming upon release from the mold.
The preferred method for injection-molded cross-linked compositions involves placing or injecting a mixture of the polymeric material and the chemical blowing agent into a hot mold that is maintained at an elevated temperature higher than the decomposition temperature of the chemical blowing agent. The hot mold is filled with the mixture, held together by a press, and opened after the chemical blowing agent is activated by the elevated temperature.
Detrimental oxidation may occur while forming foamed compositions using hot molds. For example, oxidation may occur if oxygen is present in the mold chamber while the mixture is injected into the hot mold and the chemical blowing agent is activated. Such oxidation may cause burning and discoloration of the compound resulting in a defective article. As known in the art and depending on design, it is possible to have up to approximately ninety percent of products produced by injection molding in a hot mold undergo such burning and discoloration. These discolored articles often must be discarded as "rejects," resulting in waste and expense in the manufacture of foamed compositions.
A vacuum system could be used alone to evacuate oxygen from hot molds. Such vacuum systems used alone, however, are not effective because they fail to evacuate the oxygen completely from the mold. As the hot molds wear after repeated use they fail to tightly seal and the parting lines of the molds tend to misalign thereby causing tool leakage. Such tool leakage may inadvertently cause the vacuum system to draw air containing oxygen into the mold chamber. Also, vacuum systems alone are ineffective as molding designs become more complicated due to the presence of deeper cavities which tend to entrap oxygen.
What is needed is a method to evacuate oxygen from hot molds during injection molding to reduce the possibility of oxidation of the molded article.